Not able to use the another field value in random function

I am generating the data from the Mockaroo - Random data generator

Scenario is :

I have one field Income (value can be 0 to 99999) and I have another field Net Income i want value of this field must be less then Income

So, any one can suggest me formula or regular expression for this Which support in Mockaroo.But thing that i can not use the static formula like Income - 10 or something like this